President Barack Obama assured Americans today that the National Security Agency isn't reading their emails, responding to reports that the NSA is tapping into the servers of nine leading Internet companies.

The classified Internet-monitoring program, called Prism, is designed to track the activity of potential foreign terrorists. Government officials wouldn't look at the emails of U.S. citizens and residents unless a federal judge agreed there was enough evidence to warrant this step -- similar to what happens in criminal investigations, Obama said.

Instead, the NSA is looking at telephone numbers and the durations of calls, and "sifting through this metadata" for potential leads about terrorist threats, Obama said. If government officials want to listen in on a call, they have to get approval from a federal judge, he noted.

Both classified programs are overseen by a special court created in the wake of the Sept. 11, 2001, terrorist attacks on America. Judges on this FISA Court evaluate these programs and make sure they're consistent with the Constitution, Obama said.

"They're empowered to look over our shoulder … and make sure these programs aren't being abused," he said.

Plus, both programs have been authorized by Congress, and members have been briefed about each one, the president said.

"Your duly elected representatives have been consistently informed on exactly what we're doing," Obama said.

The president hopes his comments will reassure the public that "Big Brother" isn't watching them.

But he acknowledged "there are some tradeoffs involved" in striking a balance between keeping Americans safe and protecting their privacy

Obama also said he welcomes a debate on how to balance these two goals.

He had "a healthy skepticism" about these monitoring programs when he took office, and his team "scrubbed them thoroughly," he said. But he decided "they help us prevent terrorist attacks."